Growing plants in the cooler months is a little bit like using a slow cooker -- it might take longer, but at lower temperatures the flavour really seems to set in. Kale, for example, becomes even sweeter with a frost. The science behind it says that the plant will convert its existing starches into sugars as an anti-freeze mechanism, (so harvesting our winter greens, we really are like kids in a candy shop. . .). Fall weather also gives some relief from the summer heat which can cause some greens and herbs to flower and go to seed too quickly. If you don't have a greenhouse or even cold frames, you can still extend your season with tricks like row cover, thick mulch and suitable planting varieties.